The research performed during the past year has been directed at two problems: the nature of primary afferent (hair cell-dendrite) transmission and: a continuation of the characterization of olivocochlear-hair cell neurotransmission. The first-mentioned research is ultimately aimed at identification of the chemical transmitter between mechano-sensory receptor cells and its associated primary sensory nerve. The strategy is to use the electrical activity of the primary afferent nerve as a sensitive detector of the presence of the chemical transmitter. Although ultimate interest lies in the nature of this transmitter in mammalian cochlea, the lateral line system is being studied in parallel should technical problems associated with the mammalian work prove insuperable. This work is in progress and will continue. The cyclic nucleotides have been implicated in the actions of certain transmitters. Of particular interest to us is the implication that cyclic guanosine monophosphate (GMP) is involved in muscarinic cholinergic transmission because olivo-cochlear hair cell transmission appears to be cholinergic. We have therefore determined the contents of cyclic GMP and cyclic adenosine monophosphate in the basilar membrane and spinal ligament of guinea pigs. Work in progress should decide whether destruction of hair cells and/or olivocochlear endings affects the cochlear content of these nucleotides.